Conventionally, surface ships are required to accurately acquire locations of the ship and other ships when traveling on major routes close to narrow water channels or land (or shore). Therefore, radar transceivers are equipped in the ships, while route buoys containing radar beacons (hereinafter, referred to as “the racon”) are distributed over the major routes.
FIG. 11 is a schematic diagram illustrating a conventional radar system. As shown in FIG. 11, the radar transceiver transmits a radar wave around the ship, and receives and processes a radar reflection wave (hereinafter, referred to as “the radar echo”) which is produced by the radar wave reflecting on land and/or another ship. The radar transceiver receives and processes a racon response wave which is transmitted from the racon at the same frequency as the radar wave received by the racon. Then, the radar transceiver controls, using the received racon response waves and radar echoes, to display lands, other ships, and racons on a PPI (Plan Position Indicator) screen, centering the location of the ship, as shown in FIG. 12A. JP1992-0361188A discloses one technology relevant to the above display control.
In JP04-361188A, a radar operator views the PPI screen to find out locations of the route buoys amongst other ships displayed on the PPI screen, thereby the operator can find out the location of his/her ship even in a congested ocean space.
Here, it is common to use a radar transceiver which adopted a magnetron radar. The radar transceiver transmits a radar wave at a transmission frequency determined by a magnetron oscillator. The racon which received the radar wave transmits a racon response wave at the same frequency as this radar wave, where the racon response wave is produced by modulating (compressing) a pulse for identifying a racon located in the same ocean space (hereinafter, referred to as “the identification pulse”). Thus, the radar transceiver can receive and process without discriminating between the radar echo and the racon response wave, and can display a Morse-type code indicating the racon (hereinafter, referred to as “the Morse code”) as well as land(s) and other ship(s) on the PPI screen.
In the meantime, if the above radar transceiver adopts a pulse compression radar and not the magnetron radar, the radar transceiver applies the pulse compression also to the identification pulse contained in the racon response wave. For this reason, the displayed Morse code indicating the racon is elongated in distance directions on the PPI screen, causing a problem that the Morse code is collapsed.
Specifically, the pulse compression radar transmits the radar wave containing a transmission pulse of a long time width (several microseconds to tens of microseconds) compared to the magnetron radar, and applies, to the received signal, matched filter processing (hereinafter, referred to as “the MF processing”) corresponding to the transmission pulse. Thus, the pulse width can be compressed and a signal to noise (S/N) ratio can be improved.
In this case, the radar transceiver which adopted the pulse compression radar applies the MF processing to the identification pulse contained in the racon response wave. However, since the racon response wave is a sine wave with a single frequency of which a frequency is not modulated, if the MF processing is applied to the identification pulse, the identification pulse is not pulse-compressed. Therefore, the Morse code displayed on the PPI screen is adversely elongated in the distance directions (both in a shorter distance direction and a longer distance direction), resulting in the Morse code being collapsed (see FIG. 12B).
If the Morse code is elongated in the shorter distance direction, the radar operator misrecognizes that a racon exists at an end of the displayed Morse code on the shorter distance side, and he/she misidentifies the location of the racon. In addition, another problem that other ship(s) which should originally be visible by the radar operator are hidden by the Morse code may be caused due to the elongation and collapse of the Morse code.
Therefore, when the radar transceiver adopts the pulse compression radar, it is very important to control an appropriate display of other ships and the racons on the PPI screen. The same can be said when adopting a solid-state radar other than the pulse compression radar, or when applying the above technology to transponder devices other than racons.